chore: refactor retrieval pipeline to chunk-first RRF with derived entities and slimmer eval surface.

Collapse the multi-strategy entity engine into one benchmarked chunk retrieval path, derive entities from retrieved chunks, and update consumers, docs, and clippy fixes across the workspace.

html-router/src/routes/scratchpad/handlers.rs

@@ -12,7 +12,7 @@ use crate::html_state::HtmlState;
 use crate::middlewares::{
     auth_middleware::RequireUser,
     response_middleware::{
-        template_with_headers, HtmlError, TemplateResponse, TemplateResult, ResponseResult,
+        template_with_headers, TemplateResponse, TemplateResult, ResponseResult,
     },
 };
 use common::storage::types::{